This invention pertains to dispersion strengthened metals and particularly to an improved process for internally oxidizing metal alloys. Internal oxidation is a preferred method of dispersion strengthening and generally pertains to oxidizing a solid solution metal alloy. The alloy comprises a relatively noble matrix metal having a relatively low heat or free energy of oxide formation and a solute metal having a relatively high negative heat or free energy of oxide formation. During internal oxidation, the alloy is heated under oxidizing conditions to preferentially oxidize the solute metal to form a solute metal oxide which is known in the art as in situ internal oxidation or more simply "internal oxidation." Very desirable methods of internal oxidation are shown in commonly assigned U.S. Pat. No. 3,779,714 and U.S. Pat. No. 4,077,816 and the same are incorporated herein by reference.
Preferred methods for dispersion strengthening of metals ordinarily are dependant upon power metallurgy wherein the alloy powder should have particle sizes less than about 300 microns, and preferably less than about 200 microns, for efficient internal oxidation. Internal oxidation follows a parabolic rate law wherein the time required for complete internal oxidation is proportional to the square of the diffusion distance for oxygen. In practice, doubling or tripling the particle size increases the time interval for internal oxidation four to nine times. The most practical and efficient method for producing alloy powder is by atomization of molten metal such as described in U.S. Pat. No. 2,956,304 wherein a plurality of liquid streams (water) or nitrogen gas are sprayed from nozzles to impinge upon a centrally disposed molten metal. The high pressure streams intersect the molten metal simultaneously to provide spherical shaped atomized alloy metal particles. The atomization process, however, typically produces a size range of small particles wherein as much as 50% by weight of the particles are particles larger than about 200 microns. For efficiency reasons, only the fraction of smaller particles less than about 180 microns is screened and internally oxidized. The larger particles above about 180 microns are recycled into the molten metal and reatomized.
It now has been found that the oversize alloy particles above about 180 microns can be efficiently utilized for internal oxidation by subjecting the particles to a flaking process to provide alloy flakes having a thickness less than about 180 microns and preferably to flake thickness between about 100 and 125 microns. Flattening the large atomized particles to flakes provides internal oxidation time intervals comparable to small atomized particles smaller than 180 microns. A further advantage is achieved in that the flaking of the oversize atomized particles by impact or mechanical squeezing such as rolling apparently upsets and redistributes an outer surface oxide layer which often develops during the atomization process. Such surface enriched oxide layers tend to cause weak interfaces in finished dispersion strengthened products wherein failures tend to originate along the oxide layer under heavy load and elevated temperatures. Thus, flaking of oversize atomized particles has the additional advantage in that oxide surface areas are substantially reduced and thus provides a much stronger dispersion strengthened metal product. Still further, the alloy can now be atomized at larger particle size distribution thus eliminating undesirable fines which can be detrimental in some heavy duty applications such as use in welding electrodes. These and other advantages will become more apparent by referring to the Detailed Description of the Invention.